1. Field of the Invention
This invention pertains to the field of anti-lock brake systems which are employed in wheeled vehicles to prevent the locking or skidding of the vehicle wheels when the vehicle brakes are applied. More specifically, this invention pertains to the field of anti-lock brake systems which actively control the braking force produced by the vehicle brakes in response to a sensed incipient wheel skid condition in order to produce a maximum braking effort until such a time as the possibility of a dangerous wheel skid no longer exists.
2. Description of Related Art
Various systems have been proposed for preventing the locking or skidding of the vehicle wheels when the vehicle brakes are applied. A current trend in automotive hydraulic brake system design is to provide each brake line in the brake system with access to a drain and an auxiliary pressure source. A first solenoid valve is employed between the brake line and the drain to permit controlled draining of brake fluid from the brake line when it is necessary to relieve hydraulic braking pressure in the brake line. The pressure source is selectively communicated with the brake line through a second solenoid valve to readmit the drained brake fluid into the brake line when it is necessary to increase the hydraulic braking pressure. In this manner, the two solenoid valves can be selectively controlled in order to modulate the hydraulic pressure at the wheel cylinder between a low and a high level. Such modulation of the hydraulic pressure at the wheel cylinder is generally controlled by a microprocessor and occurs independently of the hydraulic pressure in the operator controlled master cylinder. Specifically, during braking, the microprocessor continually receives inputs from wheel speed sensors and calculates the magnitudes and/or rates of change of angular deceleration of the vehicle wheels. From this information, the microprocessor determines whether or not any incipient wheel skid conditions exist. If an incipient wheel skid condition exists at any particular wheel, the microprocessor immediately isolates the brake line leading to the particular wheel from the master cylinder and then outputs a first electrical signal to energize the first solenoid valve to establish communication between the brake line and the drain. Thus, the hydraulic pressure in the wheel cylinder is immediately reduced to a low level and stays reduced until such a time as the microprocessor determines that the particular wheel is rolling again. When the wheel has regained traction (that is, when the wheel is rolling again), the microprocessor immediately outputs a second electrical signal to energize the second solenoid valve to establish communication between the pressure source and the brake line, thereby allowing the pressure source (which can typically comprise a hydraulic pump driven by an electric motor) to pump the drained brake fluid back into the brake line. Consequently, the hydraulic pressure in the wheel cylinder increases and continues to increase until such a time as the microprocessor determines that the particular wheel is skidding again. Thereafter, the microprocessor must immediately output the first electrical signal again until it determines that the particular wheel is rolling, followed by the second electrical signal, then the first, and so on. Additionally, some of the current anti-lock brake systems provide valve means for allowing the microprocessor to maintain the hydraulic pressure in the wheel cylinder constant for brief periods of time. One example of this kind of system is disclosed in U.S. Pat. No. 4,676,353, herein incorporated by reference. All in all, it is the current trend in anti-lock brake systems to modulate (i.e increase or decrease) the hydraulic pressure in the wheel cylinders of skid controlled wheels at a rate of between 7 and 20 times a second.
It has been known that the maximum possible braking force between a wheeled vehicle and, for example, a dry road surface is achieved when the slip between the decelerating wheels and the road surface is maintained at a level between about 10% and 20%. That is to say, in order to stop a wheeled vehicle in the shortest possible distance, the vehicle wheels should be retarded with a force which causes the tire surfaces on the wheels to slip (in the braking or decelerating direction) relative to the road surface by an amount (i.e a distance) equal to between about 10% and 20% of the distance travelled by the vehicle during braking.
It becomes apparent then that the current anti-lock braking systems as described above are not designed to provide the maximum possible braking of a vehicle during a skid control operation because they are not designed to maintain the wheel slip during deceleration at an amount which is between about 10% and 20%. Rather, these systems have been designed to prevent the vehicle wheels from locking completely and to ensure that a good (though not optimum) level of traction is maintained during braking.
Moreover, owing to their relative complexity, the current anti-lock brake systems have heretofore been offered only as original equipment on new vehicles. No system has been proposed which meets the needs of those consumers who have bought vehicles in the past without anti-lock brakes systems but now want to retrofit such systems to their vehicles. That is, no after-market anti-lock brake systems are currently available.
The following patent documents, incorporated herein by reference, reveal brake systems which are similar in one or more respects to the anti-lock brake system of the present invention:
______________________________________ Origin Document Number ______________________________________ U.S. 3,731,979 U.S. 4,705,323 U.S. 4,826,256 Japan 57-140263(A) Japan 61-139927(A) Japan 61-139928(A) European Pat. Office 152344(A) Fed. Rep. Germany DE-O 2154227 Fed. Rep. Germany DE-O 3318326 Fed. Rep. Germany DE-O 3441380 ______________________________________